# Higher-Dimensional Algebra VII: Groupoidification

## TL;DR

This paper introduces groupoidification, a categorification method replacing vector spaces with groupoids, and demonstrates its applications to quantum harmonic oscillators, Hecke algebras, and Hall algebras, offering new combinatorial and categorical insights.

## Contribution

It systematically develops degroupoidification and applies it to key algebraic structures, providing novel interpretations and connections in quantum algebra and categorification.

## Key findings

- Groupoidification provides a combinatorial interpretation of quantum operators.
- Degroupoidification of the A2 Hecke algebra relates to projective geometry over finite fields.
- The approach offers a new perspective on categorifying quantum groups.

## Abstract

Groupoidification is a form of categorification in which vector spaces are replaced by groupoids, and linear operators are replaced by spans of groupoids. We introduce this idea with a detailed exposition of "degroupoidification": a systematic process that turns groupoids and spans into vector spaces and linear operators. Then we present three applications of groupoidification. The first is to Feynman diagrams. The Hilbert space for the quantum harmonic oscillator arises naturally from degroupoidifying the groupoid of finite sets and bijections. This allows for a purely combinatorial interpretation of creation and annihilation operators, their commutation relations, field operators, their normal-ordered powers, and finally Feynman diagrams. The second application is to Hecke algebras. We explain how to groupoidify the Hecke algebra associated to a Dynkin diagram whenever the deformation parameter q is a prime power. We illustrate this with the simplest nontrivial example, coming from the A2 Dynkin diagram. In this example we show that the solution of the Yang-Baxter equation built into the A2 Hecke algebra arises naturally from the axioms of projective geometry applied to the projective plane over the finite field with q elements. The third application is to Hall algebras. We explain how the standard construction of the Hall algebra from the category of representations of a simply-laced quiver can be seen as an example of degroupoidification. This in turn provides a new way to categorify - or more precisely, groupoidify - the positive part of the quantum group associated to the quiver.